Filtration systems play an important role in a wide variety of industrial and commercial processes which generally increase our quality of life. High efficiency filtration systems are currently being used in numerous medical applications, including but not limited to blood filtration and the separation of microorganisms such as, bacteria and viruses from biological or other fluids (both liquids and gases). In this regard, filtration technology is also beneficial in the drug, cosmetic and beverage industries. Filters are also used to a great extent in the semiconductor and microelectronics manufacturing industry for fine clarification and for the special cleaning of liquids and gases. In addition to their role in separating materials, micro- and ultrafilters may also be used in catalytic processes to enhance chemical reactions taking place during the separation process or procedure.
A wide variety of materials having various geometries are used as filters according to existing techniques. As one may well imagine filters very broadly in composition, shape, and size with each parameter dependent upon the intended application. While filters may be manufactured front it host of materials, plastics, ceramics and metals, each having separate advantages and disadvantages, are most often used. Regardless of the material comprising the filter element, the major attributes desirable for filter elements are: (i) uniformity in pore size and pore size distribution especially in small dimensions. (ii) low pressure drop for flow of fluids, (iii) flexibility and mechanical strength to avoid collapse or tearing, and (iv) low rate of fowling and ease of cleaning. In addition some separation applications require the filter to perform in a high temperature environment or in a corrosive or "hostile" environment; consequently, the ability of the filter element to resist abrasion or shedding of particles can also bean important attribute. In this regard, metal filters are ideal candidates.
Metal filters, typically formed from wire mesh screen, have long been used for a variety of applications where relatively fine filtration capability must be combined with mechanical strength. flexibility, resistance to high temperatures and/or resistance to chemical attack. While this type of filter has many desirable characteristics, it suffers from low efficiency, for the removal of fine particles due to relatively large pore sizes of the wire mesh structure In an effort to create filters having uniform pore sizes in the range of 10 micrometers to 0.01 micrometers or less, attempts, met with limited success, have been made to alter the underlying size of the pores in a porous substrate by the application of a second and possibly subsequent layer(s) of material.
For example. Gaddis, et al., in U.S. Pat. No. 4,888,184, discloses a process for forming a filter having a metallic base. Metal oxide particles (e.g. TiO.sub.2) having a size of from 0.2 to 1.0 micrometer are drawn into a porous metal substrate, such as, stain less steel, having a pore size of from about 0.5 micrometers to about 10 micrometers and the excess metal oxide particles are then removed from the surface of the substrate. The metal oxide particles within the metal substrate are then sintered to form a filter element.
U.S. Pat. No. 4,613,369 by Koehler discloses a method for making a porous filter. A stabilized suspension of dispersed metal particles is applied to a porous metal support, such as a wire mesh screen, to infiltrate the openings in the porous metal support. Excess particles are removed from the surface of the support with a doctor blade. The support is then heated to dry the stabilization suspension of metal particles and is compressed between rollers to decrease the pore size and improve the sintering characteristics. The support is then sintered to fuse the individual metal particles to the metal support and to each other.
U.S. Pat. No. 5,364,585 by Trusov, et al., discloses a method for making a porous composite membrane. Metallic particles having a particle size of less than about 50 .mu.m are dispersed on a metallic substrate to form a sublayer wherein substantially no metallic particles are in contact with adjacent metallic particles. Subsequent to pre-sintering this sublayer, ultra-fine ceramic particles having an average particle size of less than about 200 nm are deposited on the substrate and plastically deformed by passing the substrate though a rolling mill and sintering the deformed particles to form a composite membrane.
The disadvantages associated with the techniques described above involve the plugging of the existing pores in the porous substrates by means of pressing powders into the openings and thereafter heating or annealing such powders to simply fill the pores to reduce their dimension. Due to the loose attachment of the deposited material and the tortuous flow paths created these fillers cause a high pressure drop across the filter, since pressure drop through the filter is a function of pore size, number of pores, tortuosity of the flow path and length of the flow path. Furthermore, where a filter is intended to be reusable, as opposed to simply being disposed of after time, it is necessary to clean the filter element. Cleaning a filter element is often accomplished by backflowing or backflushing a fluid (liquid or gas) through the filter or running a fluid at high shear velocity along the surface so that the retenate is dislodged. Such attempts at cleaning the filters described above can destroy or remove significant portions of a weakly deposited membrane surface.
In addition to the disadvantages discussed above, it is often desirable to form filters in a variety of shapes in order to provide large Surface areas for filtration within a small package. Fluted and/or dimpled patterns are common patterns to increase surface area. Cylindrical shapes provide maximum strength capability where a high pressure drop is anticipated. Moreover, the geometry of construction can define the strength of the element. Thus, it is desirable that filter elements having different configurations be available. The deposition techniques disclosed by Gaddis, et al., Koehler and Trusov, et al., do not readily lend themselves to the construction of filter elements in a variety of geometric shapes. Due to the flow characteristics of the deposited layers, non-uniformity of deposition can occur such that portions of the filter element will be completely closed while other portions of the filter element remain relatively open so that substantial inconsistency in filtering capabilities resides over the surface area of the filter element.
There is still a need, therefore, for filter elements and methods for fabricating the same, which have high mechanical strength, uniformity in pore size and pore size distribution, the ability of being formed in a variety of geometric shapes, and which can resist harsh or hostile environments.